SCHOTT solutions no. 2/2013 > Digital X-Ray Systems

Digital x-rays with flat image detectors open up new diagnostic possibilities, simplify difficult surgical proce- dures and improve their chances of being successful. Photo: SCHOTT

Smart Fiber Optics for Use in Modern Radiology

Sophisticated dynamic or 3-D x-rays would be impossible without using state-of-the-art flat image detectors that feature fast CMOS sensors. New RoHS-conforming fiber optic plates that offer high transmission and x-ray attenuation support this trend.

Thilo Horvatitsch

Today, 3-D x-ray devices that rotate around a patient’s jaw and produce many different individual images that can be joined together on the computer to form three-dimensional images are quite commonplace in progressive dental practices. They make it possible for implants to be aligned down to the millimeter and sometimes discover a few surprises. Both this technology and digital x-rays are rapidly gaining ground in operating rooms. Here, a series of photos are taken, which then appear on the screen as continuous image sequences – a type of live control for surgeons.

The fascinating imaging techniques that are used in modern radiology open up new diagnostic possibilities, enable difficult surgical procedures and significantly improve their chances of success. This can all be attributed to digital x-ray technologies that are replacing analog techniques. Time-consuming development of film is now a thing of the past. Instead, these images are recorded either directly or via image sensors and then digitalized so that they can be accessed and distributed very quickly by computer. Digital technology is also much more light-sensitive, has short exposure times and allows for an entire series of images to be produced. At the same time, it reduces radiation often by as much as 90 percent compared with traditional x-rays.

Advanced technology is what makes this possible. Digital x-ray devices can be divided into direct and indirect flat image detectors that record and convert the radiation from x-ray tubes into digital image signals. With direct conversion, this is done using a photoconductor that is usually made of amorphous silicon (a-Si) and can be compared with the LCD or TFT thin film technology used in displays. With indirect flat image detectors, the x-ray radiation comes into contact with a scintillator layer first and is then converted into visible light. Afterwards, image sensors perform processing much like they do in digital cameras. Today’s flat image detectors based on the a-Si technique or CCD sensors are particularly well suited for producing static x-ray images. Their reading speed is hardly fast enough for dynamic or 3-D x-rays, however. For this reason, fast, low-consumption CMOS sensors are mainly being used in these types of resource-intensive applications in smaller and, in the future, large flat image detectors. Nevertheless, these semiconductor detectors are extremely sensitive to x-ray radiation. Upstream optics that block the x-ray light that is not converted by the scintillator can protect the sensor and improve its performance.

And this is exactly what fiber optics plates do, such as the type that SCHOTT has been offering for some time. This type of plate consists of a number of extremely thin individual fibers that are aligned parallel in an extremely precise manner and are bonded together using a heat process. The plate is placed directly in front of the sensor and offers extremely good transmission of visible light and excellent x-ray attenuation. This excellent x-ray attenuation, in turn, reduces the so-called noise floor. These interference signals occur inside the sensor as a result of x-ray radiation and can overlap the light signals and make it very difficult to read them. Fiber optic plates limit this and, at the same time, increase the contrast of the x-ray image quite significantly. SCHOTT will now be introducing a new generation of these types of products under the name RFG92. These fiber optic plates are manufactured using a modified glass material that meets all of the relevant RoHS standards and does not contain substances that are harmful to the environment.

Unlike other manufacturers, SCHOTT is capable of offering monolithic square formats up to 320 millimeters in length. This offers advantages over products that consist of multiple glued fiber optic plates because the lines of glue are picked up by the image sensor and can often only be extracted by software at the expense of reading speed. Glue lines also offer no x-ray attenuation, therefore this increases the risk of noise and the sensor being damaged. Last, but not least, glued fiber optic plates are more sensitive to mechanical stresses than monolithic plates.

”We have developed an environmentally-friendly product that requires no RoHS exemption permits and fully meets today’s
market demands in terms of both its technology and its price,” explains Jörg Warrelmann. The Senior Product Manager for
Medical at SCHOTT Lighting and Imaging sees areas of application in mainly large-format flat image detectors such as those that are used to produce large cardiovascular images and in dynamic 3-D or mammogram x-rays in the future. Further market potential also exists in a completely different field: industrial technical in-line inspections during which parts are scanned at extremely high speeds.

With indirect flat image detectors, the x-ray radiation penetrates through the object to be examined, encounters a scintillator layer and is then converted into visible light. If CMOS image sensors are to perform the digital processing, then a fiber optic plate (FOP) must be used as an intermediate layer. This allows the visible light that has been converted to pass through, but blocks out x-ray radiation and thus protects the sensitive sensors. The plate consists of many individual fibers aligned in parallel through which the light is guided during socalled total reflection. Source: SCHOTT/wissen + konzepte